The present invention relates to machine vision systems, and more particularly, to a modular focus system that enables an image-based ID, or code, reader to be operated as a manual focus reader and as a variable focus reader.
Machine vision systems use image acquisition devices such as cameras to capture images of various objects. These images are processed according to any number of decoding algorithms to obtain information about each of the imaged objects. Various identification and/or decision-making functions, such as for parts inspection, inventory, quality control, and the like, are made based on the obtained information.
An image-based code reader is a type of machine vision system. Images acquired for code reading, also known as direct part mark (“DPM”) or label-based identification, include one or more 1D (i.e., a barcode) or 2D s (i.e., a Data Matrix code) symbols located on an object. The code reading process includes repeatedly introducing an object marked with either 1D or 2D codes within a field of view of an image acquisition sensor (e.g., CMOS camera, CCD, etc.), illuminating the codes, and acquiring images, i.e., image data, containing the codes. Each code contains a set of predetermined patterns from which an attached data processor, such as a computer, can derive useful information about the object (e.g., serial number, type, model, price, etc.).
The image data acquired by the code reader, however, must be sufficiently focused for effective and repeatable processing. To this end, image-based code readers may be constructed as fixed focus, manual focus, and/or variable focus readers. Regardless of the reader type, a decodable image is obtained when the image data is sufficiently focused onto an image acquisition sensor arranged within a focal plane of the reader.
Fixed focus code readers have a lens that is not adjustable and therefore, are the most basic type of image-based code reader. The lens is fixed in place relative to an image sensor after undergoing a factory focus process to account for the variations of each lens. When installed, the target distance, i.e., the distance from the lens to the target object is dictated by the focal length of the reader. Understandably, fixed focus readers are very useful when the target distance is relatively constant or low resolution image data is acceptable. However, they become much less suitable when the target distance varies, such as with different-sized target objects, resulting in degraded images that cannot be reliably decoded. Although a fixed focus code reader may be repositioned to obtain a focused image or multiple readers may be installed at different target distances, neither solution is particularly desirable due to the increased complexity and associated costs.
Manual focus code readers overcome some of the drawbacks of fixed focus code readers. Manual focus code readers are similar to fixed focus readers but provide a limited ability to focus the reader by adjusting the lens instead of moving the reader. By way of background, a commercially available fixed-mount, image-based code reader 2 is illustrated in FIG. 5. The code reader 2 has an optical assembly 4 including a lens 6 fixedly mounted within a lens barrel 8. The lens 6 focuses image data, i.e., reflected light from an image, onto an image plane 10 and an image sensor 20 aligned with the plane 10. The reader 2 includes hardware (not shown) encoded with software that is configured to, among other things, control illumination, acquire image data, and process/decode the acquired image data into usable information.
The code reader 2 further includes a lens mount assembly 12 that supports the lens barrel 8 and a focus ring 14 for manually setting the focus position of the code reader 2. These components are assembled about an optical axis 16 extending between a target object 18 (I.e., object to be imaged) and the image sensor 20. The sensor 20 is positioned about the intersection of the optical axis 16 and the image plane 10 such that the image data is focused by the lens 6 onto the sensor 20.
A distal end of the lens barrel 8 is secured via a threaded connection with a receptacle 22 formed within the lens mount assembly 12. A proximal end of the lens barrel 8 has the focus ring 14 fixedly secured thereto. Because of the threaded connection, manual rotation of the focus ring 14 drives the lens barrel 8 further into or out of the receptacle 22. This rotation is translated into an axial movement between the lens 6 and the image sensor 20.
The lens mount assembly 12 is secured within a housing 24 and covered by a clear lens cover 26 to protect the optical and hardware components from environmental conditions such as dust and moisture. The cover 26 and a sealing gasket (not shown) are secured to the housing 24 with screws 40. A cable 28 provides a communication link between the code reader 2 and another device, e.g., a host computer running a database application. The computer may generate a trigger signal for the code reader 102 and receive image data. The cable 28 also supplies power to the code reader 2.
The manual focus code reader 2 is designed to be operated in one of a number of predetermined focus positions, each having a unique focal length and corresponding code reading distance range. Each focus position is indicated with an appropriate marking 30 (e.g., “40”, “65”, and “105”) on the focus ring 14, indicating the optimal reading distances, in millimeters, set by the manufacturer. Each focus position has an corresponding slot 32 formed in the periphery of the focus ring 14. The slots 32 are spaced about the focus ring 14 and receive a locking tab 34 formed in the lens cover 26. The angular position of each slot 32 is determined by the corresponding predetermined focal length, i.e., the axial distance between the lens 6 and the image sensor 20.
The focus ring 14 further includes a mechanical stop 36 to prevent the lens 6 from being rotated more than one complete revolution. The stop 36 is formed as a protrusion in the focus ring 14 that presses against a focus selector tab 38 when the focus ring 14 is at either end of the focusing range. The stop 36 limits axial movement of the lens 6 relative to the image sensor 20 to just less than one full rotation of the lens barrel 8.
To set the focus position of the reader 2, the focus ring 14 is manually adjusted to align a desired focus position setting 30 with the focus position selector tab 38. The locking tab 34 prevents the lens barrel 8 from rotating when the cover 26 is on, thus ‘locking’ the lens 6 into one of the pre-established focus positions. The cover 26 can only be secured to the housing 24 when the pin 34 is aligned with and received by one of the focus position slots 32.
Although manual focus code readers 2 provide some focusing flexibility as discussed above, changing the focus position of a code reader 2 is an involved and time consuming process. Further, the proper tools are needed to change the focus position of such a code reader 2. Still further, the lens 6 and other internal components are potentially exposed to environmental conditions including dust, dirt, water, chemicals, and the like while the lens cover 26 is removed.
Variable focus code readers overcome many of the drawbacks of manual and fixed focus code readers. As used herein, the term “variable focus code reader” refers to code readers with an electrically controllable focus system and may be part of an autofocus system. Existing variable focus code readers have very small electromechanical components such as stepper or piezo-motors that move the lens with respect to the image sensor. However, these readers have a number of expensive electromechanical components, are prone to breakage, and have only a limited number of cycles. Further, conventional variable focus code readers may not have the required optical tolerances due to friction between the lens components, may consume a large amount of power, and have a low operating speed. As such, no known code readers have a modular focus system with interchangeable lens attachments such that a single code reader to be operated as a fixed focus, manual focus, or variable focus reader depending on which lens attachment is secured to the reader without the aforementioned drawbacks.
It is therefore desirable to have a focus system for a machine vision component, such as the code reader 2, that combines the aforementioned focusing systems and overcomes the aforementioned drawbacks. It is further desirable to have a fixed or manual focus code reader that can easily be transformed into a variable focus code reader with the addition of a variable focus “add-on” or “accessory option” lens attachment. It is still further desirable to have a compact modular variable focus system with no moving parts and that does not expose the interior of the housing to the surrounding environment.